Introduction

Here's a small quick FAQ on design patterns in Q and A format. In this section we will cover factory , abstract factory , builder , prototype , shallow and deep prototype , singleton and command patterns.

You can read my other design pattern FAQ sections of the same in the below links :-

Design patterns are documented tried and tested solutions for recurring problems in a given context. So basically you have a problem context and the proposed solution for the same. Design patterns existed in some or other form right from the inception stage of software development. Let’s say if you want to implement a sorting algorithm the first thing comes to mind is bubble sort. So the problem is sorting and solution is bubble sort. Same holds true for design patterns.

Which are the three main categories of design patterns?

There are three basic classifications of patterns Creational, Structural, and Behavioral patterns.

Creational Patterns

• Abstract Factory:- Creates an instance of several families of classes • Builder: - Separates object construction from its representation • Factory Method:- Creates an instance of several derived classes • Prototype:- A fully initialized instance to be copied or cloned • Singleton:- A class in which only a single instance can exist

Note: - The best way to remember Creational pattern is by remembering ABFPS (Abraham Became First President of States).Structural Patterns

• Mediator:-Defines simplified communication between classes.• Memento:-Capture and restore an object's internal state. • Interpreter:- A way to include language elements in a program.• Iterator:-Sequentially access the elements of a collection. • Chain of Resp: - A way of passing a request between a chain of objects.• Command:-Encapsulate a command request as an object. • State:-Alter an object's behavior when its state changes. • Strategy:-Encapsulates an algorithm inside a class. • Observer: - A way of notifying change to a number of classes. • Template Method:-Defer the exact steps of an algorithm to a subclass. • Visitor:-Defines a new operation to a class without change.

Factory pattern is one of the types of creational patterns. You can make out from the name factory itself it’s meant to construct and create something. In software architecture world factory pattern is meant to centralize creation of objects. Below is a code snippet of a client which has different types of invoices. These invoices are created depending on the invoice type specified by the client. There are two issues with the code below:-

First we have lots of ‘new’ keyword scattered in the client. In other ways the client is loaded with lot of object creational activities which can make the client logic very complicated.

Second issue is that the client needs to be aware of all types of invoices. So if we are adding one more invoice class type called as ‘InvoiceWithFooter’ we need to reference the new class in the client and recompile the client also.

Figure: - Different types of invoice

Taking these issues as our base we will now look in to how factory pattern can help us solve the same. Below figure ‘Factory Pattern’ shows two concrete classes ‘ClsInvoiceWithHeader’ and ‘ClsInvoiceWithOutHeader’.

The first issue was that these classes are in direct contact with client which leads to lot of ‘new’ keyword scattered in the client code. This is removed by introducing a new class ‘ClsFactoryInvoice’ which does all the creation of objects.

The second issue was that the client code is aware of both the concrete classes i.e. ‘ClsInvoiceWithHeader’ and ‘ClsInvoiceWithOutHeader’. This leads to recompiling of the client code when we add new invoice types. For instance if we add ‘ClsInvoiceWithFooter’ client code needs to be changed and recompiled accordingly. To remove this issue we have introduced a common interface ‘IInvoice’. Both the concrete classes ‘ClsInvoiceWithHeader’ and ‘ClsInvoiceWithOutHeader’ inherit and implement the ‘IInvoice’ interface.

The client references only the ‘IInvoice’ interface which results in zero connection between client and the concrete classes ( ‘ClsInvoiceWithHeader’ and ‘ClsInvoiceWithOutHeader’). So now if we add new concrete invoice class we do not need to change any thing at the client side.

In one line the creation of objects is taken care by ‘ClsFactoryInvoice’ and the client disconnection from the concrete classes is taken care by ‘IInvoice’ interface.

Figure: - Factory pattern

Below are the code snippets of how actually factory pattern can be implemented in C#. In order to avoid recompiling the client we have introduced the invoice interface ‘IInvoice’. Both the concrete classes ‘ClsInvoiceWithOutHeaders’ and ‘ClsInvoiceWithHeader’ inherit and implement the ‘IInvoice’ interface.

Figure :- Interface and concrete classes

We have also introduced an extra class ‘ClsFactoryInvoice’ with a function ‘getInvoice()’ which will generate objects of both the invoices depending on ‘intInvoiceType’ value. In short we have centralized the logic of object creation in the ‘ClsFactoryInvoice’. The client calls the ‘getInvoice’ function to generate the invoice classes. One of the most important points to be noted is that client only refers to ‘IInvoice’ type and the factory class ‘ClsFactoryInvoice’ also gives the same type of reference. This helps the client to be complete detached from the concrete classes, so now when we add new classes and invoice types we do not need to recompile the client.

Abstract factory expands on the basic factory pattern. Abstract factory helps us to unite similar factory pattern classes in to one unified interface. So basically all the common factory patterns now inherit from a common abstract factory class which unifies them in a common class. All other things related to factory pattern remain same as discussed in the previous question.

A factory class helps us to centralize the creation of classes and types. Abstract factory helps us to bring uniformity between related factory patterns which leads more simplified interface for the client.

Figure: - Abstract factory unifies related factory patterns

Now that we know the basic lets try to understand the details of how abstract factory patterns are actually implemented. As said previously we have the factory pattern classes (factory1 and factory2) tied up to a common abstract factory (AbstractFactory Interface) via inheritance. Factory classes stand on the top of concrete classes which are again derived from common interface. For instance in figure ‘Implementation of abstract factory’ both the concrete classes ‘product1’ and ‘product2’ inherits from one interface i.e. ‘common’. The client who wants to use the concrete class will only interact with the abstract factory and the common interface from which the concrete classes inherit.

Figure: - Implementation of abstract factory

Now let’s have a look at how we can practically implement abstract factory in actual code. We have scenario where we have UI creational activities for textboxes and buttons through their own centralized factory classes ‘ClsFactoryButton’ and ‘ClsFactoryText’. Both these classes inherit from common interface ‘InterfaceRender’. Both the factories ‘ClsFactoryButton’ and ‘ClsFactoryText’ inherits from the common factory ‘ClsAbstractFactory’. Figure ‘Example for AbstractFactory’ shows how these classes are arranged and the client code for the same. One of the important points to be noted about the client code is that it does not interact with the concrete classes. For object creation it uses the abstract factory ( ClsAbstractFactory ) and for calling the concrete class implementation it calls the methods via the interface ‘InterfaceRender’. So the ‘ClsAbstractFactory’ class provides a common interface for both factories ‘ClsFactoryButton’ and ‘ClsFactoryText’.

Figure: - Example for abstract factory

We will just run through the sample code for abstract factory. Below code snippet ‘Abstract factory and factory code snippet’ shows how the factory pattern classes inherit from abstract factory.

Figure: - Abstract factory and factory code snippet

Figure ‘Common Interface for concrete classes’ how the concrete classes inherits from a common interface ‘InterFaceRender’ which enforces the method ‘render’ in all the concrete classes.

Figure: - Common interface for concrete classes

The final thing is the client code which uses the interface ‘InterfaceRender’ and abstract factory ‘ClsAbstractFactory’ to call and create the objects. One of the important points about the code is that it is completely isolated from the concrete classes. Due to this any changes in concrete classes like adding and removing concrete classes does not need client level changes.

Builder falls under the type of creational pattern category. Builder pattern helps us to separate the construction of a complex object from its representation so that the same construction process can create different representations. Builder pattern is useful when the construction of the object is very complex. The main objective is to separate the construction of objects and their representations. If we are able to separate the construction and representation, we can then get many representations from the same construction.

Figure: - Builder concept

To understand what we mean by construction and representation lets take the example of the below ‘Tea preparation’ sequence.

You can see from the figure ‘Tea preparation’ from the same preparation steps we can get three representation of tea’s (i.e. Tea with out sugar, tea with sugar / milk and tea with out milk).

Figure: - Tea preparation

Now let’s take a real time example in software world to see how builder can separate the complex creation and its representation. Consider we have application where we need the same report to be displayed in either ‘PDF’ or ‘EXCEL’ format. Figure ‘Request a report’ shows the series of steps to achieve the same. Depending on report type a new report is created, report type is set, headers and footers of the report are set and finally we get the report for display.

Figure: - Request a report

Now let’s take a different view of the problem as shown in figure ‘Different View’. The same flow defined in ‘Request a report’ is now analyzed in representations and common construction. The construction process is same for both the types of reports but they result in different representations.

Figure: - Different View

We will take the same report problem and try to solve the same using builder patterns. There are three main parts when you want to implement builder patterns.

• Builder: - Builder is responsible for defining the construction process for individual parts. Builder has those individual processes to initialize and configure the product.• Director: - Director takes those individual processes from the builder and defines the sequence to build the product.• Product: - Product is the final object which is produced from the builder and director coordination.

First let’s have a look at the builder class hierarchy. We have a abstract class called as ‘ReportBuilder’ from which custom builders like ‘ReportPDF’ builder and ‘ReportEXCEL’ builder will be built.

Figure: - Builder class hierarchy

Figure ‘Builder classes in actual code’ shows the methods of the classes. To generate report we need to first Create a new report, set the report type (to EXCEL or PDF) , set report headers , set the report footers and finally get the report. We have defined two custom builders one for ‘PDF’ (ReportPDF) and other for ‘EXCEL’ (ReportExcel). These two custom builders define there own process according to the report type.

Figure: - Builder classes in actual code

Now let’s understand how director will work. Class ‘clsDirector’ takes the builder and calls the individual method process in a sequential manner. So director is like a driver who takes all the individual processes and calls them in sequential manner to generate the final product, which is the report in this case. Figure ‘Director in action’ shows how the method ‘MakeReport’ calls the individual process to generate the report product by PDF or EXCEL.

Figure: - Director in action

The third component in the builder is the product which is nothing but the report class in this case.

Figure: - The report class

Now let’s take a top view of the builder project. Figure ‘Client,builder,director and product’ shows how they work to achieve the builder pattern. Client creates the object of the director class and passes the appropriate builder to initialize the product. Depending on the builder the product is initialized/created and finally sent to the client.

Figure: - Client, builder, director and product

The output is something like this. We can see two report types displayed with their headers according to the builder.

Prototype pattern falls in the section of creational pattern. It gives us a way to create new objects from the existing instance of the object. In one sentence we clone the existing object with its data. By cloning any changes to the cloned object does not affect the original object value. If you are thinking by just setting objects we can get a clone then you have mistaken it. By setting one object to other object we set the reference of object BYREF. So changing the new object also changed the original object. To understand the BYREF fundamental more clearly consider the figure ‘BYREF’ below. Following is the sequence of the below code:

In the first step we have created the first object i.e. obj1 from class1.

In the second step we have created the second object i.e. obj2 from class1.

In the third step we set the values of the old object i.e. obj1 to ‘old value’.

In the fourth step we set the obj1 to obj2.

In the fifth step we change the obj2 value.

Now we display both the values and we have found that both the objects have the new value.

Figure :- BYREf

The conclusion of the above example is that objects when set to other objects are set BYREF. So changing new object values also changes the old object value.

There are many instances when we want the new copy object changes should not affect the old object. The answer to this is prototype patterns.

Lets look how we can achieve the same using C#. In the below figure ‘Prototype in action’ we have the customer class ‘ClsCustomer’ which needs to be cloned. This can be achieved in C# my using the ‘MemberWiseClone’ method. In JAVA we have the ‘Clone’ method to achieve the same. In the same code we have also shown the client code. We have created two objects of the customer class ‘obj1’ and ‘obj2’. Any changes to ‘obj2’ will not affect ‘obj1’ as it’s a complete cloned copy.

There are two types of cloning for prototype patterns. One is the shallow cloning which you have just read in the first question. In shallow copy only that object is cloned, any objects containing in that object is not cloned. For instance consider the figure ‘Deep cloning in action’ we have a customer class and we have an address class aggregated inside the customer class. ‘MemberWiseClone’ will only clone the customer class ‘ClsCustomer’ but not the ‘ClsAddress’ class. So we added the ‘MemberWiseClone’ function in the address class also. Now when we call the ‘getClone’ function we call the parent cloning function and also the child cloning function, which leads to cloning of the complete object. When the parent objects are cloned with their containing objects it’s called as deep cloning and when only the parent is clones its termed as shallow cloning.

Punch :- Create a single instance of object and provides access to this single instance via a central point.

There are situations in project where we want only one instance of the object to be created and shared between the clients. For instance let’s say we have the below two classes, currency and country.

These classes load master data which will be referred again and again in project. We would like to share a single instance of the class to get performance benefit by not hitting the database again and again.

The private constructor is important so that clients cannot create object of the class directly. If you remember the punch, the main intention of this pattern is to create a single instance of the object which can be shared globally, so we do not want to give client the control to create instances directly.

Command pattern allows a request to exist as an object. Ok let’s understand what it means. Consider the figure ‘Menu and Commands’ we have different actions depending on which menu is clicked. So depending on which menu is clicked we have passed a string which will have the action text in the action string. Depending on the action string we will execute the action. The bad thing about the code is it has lot of ‘IF’ condition which makes the coding more cryptic.

Figure: - Menu and Commands

Command pattern moves the above action in to objects. These objects when executed actually execute the command. As said previously every command is an object. We first prepare individual classes for every action i.e. exit, open, file and print. Al l the above actions are wrapped in to classes like Exit action is wrapped in ‘clsExecuteExit’ , open action is wrapped in ‘clsExecuteOpen’, print action is wrapped in ‘clsExecutePrint’ and so on. All these classes are inherited from a common interface ‘IExecute’.

Figure: - Objects and Command

Using all the action classes we can now make the invoker. The main work of invoker is to map the action with the classes which have the action. So we have added all the actions in one collection i.e. the arraylist. We have exposed a method ‘getCommand’ which takes a string and gives back the abstract object ‘IExecute’. The client code is now neat and clean. All the ‘IF’ conditions are now moved to the ‘clsInvoker’ class.

It is a good article. Thanks for sharing.
However, the missing information in the Builder Pattern is making very difficult to understand/implement. It would be better if you kindly add the complete code.

nice article. but i can't see any major difference between factory pattern and abstract factory pattern. the only difference is how it is implemented. otherwise both will resolve the same problem, is that correct ?

Anti Patterns : an anti-pattern (or antipattern) is a pattern used in social or business operations or software engineering that may be commonly used but is ineffective and/or counterproductive in practice

Looking at the example code of Abstract Factory pattern, the classes “clsFactoryButton” and “clsFactoryTextBox” inherit “clsAbstractFactory” class. As, no abstract method is provided in the “clsAbstractFactory” class, what is the need to inherit this class by the clsFactoryTextBox” inherit “clsAbstractFactory” classes. The example would run fine without inheritance also. Explain explain.

Thanks for the article, it proved to be very educational. However, I think the code examples for the Factory Pattern and the Abstract Factory are incorrect. If by Factory Pattern you are referring to the GoF Factory Method Pattern, then the definition requires that we "

Quote:

Define an interface for creating an object, but let subclasses decide which class to instantiate.

. What you describe is simply a static factory method also referred to as the Simple Factory[^]. Your code example handles the creation of the concrete objects through parametrization and not through subclassing. Additionally, the Factory Method Pattern also calls for the client code to address an abstract Factory as opposed to a concrete implementation of the Factory. More importantly though, the Abstract Factory pattern is a natural extension of the Factory Method for a family of like or dependent objects. The difference is not the extra level of indirection achieved through the use of a Factory interface but rather the encapsulation of the dependency of objects within a family group.

I apologize if the intension of the first section was not to describe the Factory Method, but if so, then I think it would help readers to add a section for the Factory Method for completeness.

I write article to get these kind of inputs. Your comment is absolutely on track and i am going to update the article based on this comment. This will give a complete picture of Simple Factory ,Factory method and Abstract factory.

Thanks awesome article. I've been looking for a 'cut-to-the-chase' explanation of design patterns, and this has some good practical examples.
I'm using WPF and MVVM approach. We are using the ICommand interface for implementing our commands using Prism. You might want to include a public virtual bool CanExecute() in IExecute, which executes logic to decide whether the command can be executed or not.

I think it is incorrect explanation of single ton pattern in this article.

Single pattern will have private constructor but the class will be having one public static method called GetInstance() and that method will return instance which refers to the single object which will be managed internally. And the class can have many oublic instance methods and they can be invoked by object instance by the client.See example below:

Actually if we invoke static methods of a class as in this tutorial then there will be actually no object instance will be referred.

Small correction in my example below. You can see there is null reference check and object will be created only first time and only one object will be created in the lifetime of the application. On all the subsequent request to GetInstance() method, the existing object reference will be sent. Please let me know your views.

So all the clients of SinglePattern class will be using same object reference to call GetInput() method and it implements actual singleton pattern.

I agree you have more improvised implementation. What i suggested at the top is a basic one and did not want to confuse people. In your code the SinglePattern class is just a static factory class which helps generates singleton objects. So in other words you have a normal class you can convert the same in to singleton class by using a factory wrapper as you suggested. The SinglePattern class can probably create 10 other objects which are of different classes. So my view of that class is that its just punches out static objects.

The whole point is one instance of the class should be shared it either comes from a singleton factory as you have made or its the class himself as i have made.

From this article: "There are situations in a project where we want only one instance of the object to be created and shared between the clients."

The above definition is absolutely correct. And you say only one instance of the object created and shared between the clients.

But confusing point is that from your example : Hit() method declared as a static and client calling it using class name will not form a singleton pattern because you have not created any object refernce to call that method. I should say it as a incorrect example for singleton pattern.

Shivprasad koirala wrote:

should be shared it either comes from a singleton factory as you have made or its the class himself as i have made.

In the above comment, you said "its the class himself as i have made.". It is incorrect because from from OOPS world, Hit() method is a static and no object of the parent class will be created when calling Hit() method. So where at all does the singleton pattern come into picture here?

I read through that article what he said is right. Well i seriously need to update the singleton sample. Even if the class is there only single instances of the variables are created and not of the classes.

No way the above example is wrong thats it....No more postings to this thread...

Oh yes you said right there is one instance created when the first static method is called of a class.

Frankly i am not a great guy , but i think i am a good author. I do not intentionally write wrong things. I write what the GURU's of that trade say in a simplified manner. The above code concept is taken from GOF's book. So i think they are right. Said and done DP is such a huge topic and has so many thin lines we can not conclude one is right and the other is wrong.

Agile brings us to the importance of singletons. In the code example of the article it tells us to make the CTOR private. One question then is how does this class get instanciated? The class itself is not static rather the methods are, so the class itself doesn't exist until it is created. The methods are callable but this isn't really a singleton object, it's a static set of methods as shown.

from my exmple, when you call SinglePattern.GetInstance() method for the first time, object will be created. On the subsequent request for the same method will use the existing object. I hope this clarifies your doubt. I am not really sure how does that related to Agile methodology. . Thanks.

Agile is about refactoring. Refactoring continually refines the way we do things. It brings us right into the concept of singletons because if we have code all over the place that is consistantly newing up classes many of them the same classes, Agile says to refactor duplicate code into one time, one place (common OOP principal). So if we are really looking into the reasons for newing up classes and honestly assess a better way, we arrive at the singleton pattern. Not that singleton's can't produce problems like "taking up memory when not needed". There are problems but, I've found that using Singleton Factories, actually speed up my application when swapping views. I like fast and responsive applications as do the users. As such, Agile showed me all the areas that I could benefit with the singleton, that's the link for me.

I have had some problems with the dreaded "Child already belongs to parent, Disconnect it first" issues, but once resolved, my apps. have been blaziningly fast when swapping views due to the singleton.

Yes. The article is very good, i'm already reading the second one. But abstract factory pattern really needs more detailed explanation. It's hard to understand the global difference between factory and abstract factory patterns...